PLANT RECORDS OF THE ROCKS SEWARD 367 



most abundant plants in the chert, appropriately named Rhynia, 

 grew to a height of a few inches; its leafless cylindrical stems were 

 borne on a creeping underground rhizome without roots, and some 

 of the slender branches were crowned with cylindrical spore-cap- 

 sules. Cross sections of the erect stems reveal an amazingly perfect 

 preservation; a strand of comparatively thick-walled conducting 

 tissue lies in the center of a mass of more delicate cells in which the 

 remains of the living contents are clearly preserved in silica. 

 Rhynia differs in some respects from all living plants; it is one of 

 the simplest and most primitive of all known members of the vege- 

 table kingdom which are provided with a conducting strand of 

 woody tissue. With other Devonian types it has been assigned to 

 a group known as the Psilophytales, which includes among other 

 plants the genus Psilophytoii described by Dawson and so named 

 b}^ him because of its resemblance in certain characters to the 

 existing PsilotuTn of the southern Tropics. 



Another of the Scottish plants, Asteroxylon., differed from Rhynia 

 in having crowded scalelike leaves on the main stem and lower 

 branches, while at the tips of more slender leafless branches were 

 borne spore capsules. AsteroxyJon, like Rhyjiia, was rootless. It 

 was slightly larger; in its leafy stems and in the form of the con- 

 ducting strand it is comparable to living species of Lycopodium. 

 It is noteworthy that some of these Devonian plants resemble more 

 than one type of living plants; Rhynia and Asteroxylon between 

 them bear some resemblance to mosses, to Psilotum, and to Lycopo- 

 dium.\ the spore capsules of Asteroxylon are not unlike those of 

 some of the oldest known ferns. Thus we find in a single extinct 

 plant or in closely allied plants points of contact with more than 

 one living type; it would seem that such association in one individ- 

 ual of characters now distributed among different families or groups 

 may be indicative of the common ancestry of plants that are now 

 far apart. Rhynia and Asteroxylon are selected as examples of 

 Devonian plants because of their exceptionally good state of preser- 

 vation ; we are able to picture them as living plants and to describe 

 their minute structure with a completeness which makes it difficult 

 for us to remember that the}^ were members of a flora which existed 

 about 300,000,000 years ago. Many other Devonian plants are 

 known from North America, Norway, Scotland, Germany, and else- 

 where, and it is j^ossible to obtain a general impression of the out- 

 standing features of a vegetation characteristic of the earlier stages 

 of the Devonian period. The majority of the plants were leafless, 

 or provided only with small appendages in place of the usual flat 

 foliage leaves; some reached a length of several feet and in general 

 plan of construction bore a certain resemblance to ferns, though true 



